1. Field of the Invention
The invention concerns optical signal transmission and in particular wavelength division demultiplexing. To be more precise, the invention concerns an optical filter tuned by rotation and comprising a Fabry-Perot type interferometer disposed in a collimated beam between an input optical fiber and an output optical fiber.
2. Description of the Prior Art
FIG. 1 shows the receive part of an optical transmission system comprising optical filters carrying out wavelength division demultiplexing. This figure and the following description explain the context in which a Fabry-Perot type interferometer can be used.
An optical fiber 10 conveys a plurality of optical signals with respective wavelengths .lambda..sub.1, .lambda..sub.2, . . . .lambda..sub.n. Each optical signal is frequency or amplitude modulated by a different signal. A star coupler 11 is connected to the end of the optical fiber 10 and feeds the received optical signal to the same number of output fibers 12.sub.1 through 12.sub.n each connected to the input of a tunable etalon filter 13.sub.1 through 13.sub.n. Each filter 13.sub.1 through 13.sub.n is tuned to a different wavelength of the optical signal conveyed by the optical fiber 10 and, by analogy, would constitute a bandpass filter in the electrical domain. The filter 13.sub.1 is thus tuned to the wavelength .lambda..sub.1, the filter 13.sub.2 to the wavelength .lambda..sub.2 and the filter 13.sub.n to the wavelength .lambda..sub.n. Each filter feeds a filtered optical signal at a given wavelength to an optical receiver 14.sub.1 through 14.sub.n. The optical receivers 14.sub.1 through 14.sub.n comprise a detector photodiode, for example, and means for shaping the detected signal producing a voltage proportional to the luminous intensity detected by the associated photodiode.
Each of the filters 13.sub.1 through 13.sub.n may comprise a Fabry-Perot type interferometer tuned to a given optical wavelength and thus filtering the optical channel at this wavelength. In this way the filters carry out wavelength division demultiplexing.
The article "INLINE TUNABLE ETALON FILTER FOR OPTICAL CHANNEL SELECTION IN HIGH DENSITY WAVELENGTH DIVISION MULTIPLEXED FIBER SYSTEMS" by A. FRENKEL and C. LIN, BELL COMMUNICATIONS RESEARCH, ELECTRONIC LETTERS, Feb. 4, 1988, vol. 24, n.degree.3 describes a Fabry-Perot interferometer of this kind applied to optical signal filtering. This interferometer is rotated to select one optical channel (one wavelength) from those constituting the input optical signal conveyed by an input optical fiber.
FIG. 2 in a diagram showing the theory of a rotation tuned optical filter of this kind using a Fabry-Perot type interferometer disposed between two coaxial optical fibers. The optical filter is the filter 13.sub.1 from FIG. 1, for example.
An input optical fiber 20 carries a composite signal, i.e. a plurality of optical signals at different wavelengths, this fiber being the fiber 12.sub.1 from FIG. 1, for example. The input fiber 20 has a silica core with a small enough diameter to transmit light in one mode only (monomode transmission). The end of the fiber 20 is glued to a collimator lens 21 producing a light beam parallel to the main axis 25 of the filter. A Fabry-Perot etalon 22 centered on the axis 25 is free to rotate about one of its axes which is not colinear with the wave vector to modify the optical path difference between two beams leaving the etalon 22 which is a thin plate of silica, for example, with a reflection treatment on both sides to meet predefined optical criteria.
The wavelength of the optical signal leaving the etalon 22 depends on the etalon rotation angle .alpha.. A focussing lens 23 coaxial with the collimator lens 21 on the axis 25 concentrates the optical signal that it receives onto the core of an output optical fiber 24 which is usually identical to the input fiber 20. The focussing lens 23 concentrates the optical energy from the etalon 22 onto the core of the output optical fiber 24. The amount of energy integrated by the core of the output optical fiber 24 depends, among other things, on the numerical aperture of the fiber 24.
The maximum number of different wavelengths (channels) that can be filtered using a rotated interferometer depends, among other things, on the structure of the interferometer, i.e. its thickness and the surface treatment. The parallelism of the surfaces, their roughness and their flatness are also important. Allowance is also made for the numerical aperture of the output optical fiber 24.
An etalon consisting of a plate with treated surfaces is advantageously used in a static mode whereby each etalon of an optical transmission system has a given angular position in order to filter a particular optical channel. An etalon plate also shows much less parameter spread than Fabry-Perot filters comprising two airspaced semi-reflecting plates. This latter type of filter is preferably used in a dynamic mode by displacing one plate relative to the other and requires a complex position control device which among other things must guarantee that the two semi-reflecting plates forming the cavity are perfectly parallel.
The rotation tuned Fabry-Perot interferometer may also comprise two fixed reflecting plates separated by a thin layer of air. In this case the reflective surface plates are mounted in a drum to ensure that they are parallel.
A Fabry-Perot type interferometer is characterized, among other things, by its free spectral range expressed in wavelengths, for example. The free spectral range is the distance between two transmission peaks of the interferometer. FIG. 3 shows these transmission peaks.
FIG. 3 shows the characteristics of a rotation tuned Fabry-Perot interferometer, for example. The wavelength is plotted on the abscissa axis and the intensity ratio I/I.sub.0 on the ordinate axis. I.sub.0 is the total optical intensity of the input optical signal and I is the intensity of the optical signal at the filter output.
This characteristic comprises a succession of transmission peaks with maximal intensity I.sub.1 and minimal intensity I.sub.2 and two adjacent transmission peaks are separated by a free spectral range representing a given wavelength difference. Two wavelengths of the input optical signal must be separated by at least the free spectral range (FSR) for the input optical signal to be filtered. The FSR expressed as a frequency is given by the equation: ##EQU1## in which c is the velocity of light, n is the refractive index of the etalon, L is the thickness of the etalon and .theta.r is the angle of refraction of the optical signal inside the etalon. This angle of refraction depends on the angle of incidence of the input optical signal light ray and therefore on the interferometer rotation angle. If the rotation angle increases the angle of refraction also increases, cos .theta.r decreases and the FSR (expressed in wavelengths) decreases. For a given input signal spectrum the output signal frequency therefore increases.
It can thus be seen that varying the angle of refraction amounts to varying the free spectral range. If the angle increases the FSR (expressed in wavelengths) decreases and the transmission peaks move towards the lower wavelengths.
The normalized intensity depends on the phase or the wavelength. In the case of a perfect Fabry-Perot interferometer (optimal parallelism, flatness and roughness) illuminated by a plane wave, it is expressed by the Airy function. This function is 2.pi. periodic and extends across all of the spectrum. Thus displacement by one free spectral range covers all the input signal spectrum. For this reason the tunability of an optical filter comprising a Fabry-Perot type interferometer is defined by the range of wavelengths needed to move from one transmission peak to the next.
Another interferometer parameter is the ratio of the FSR to the spectral bandwidth (transmission peak width at mid-height).
Finally, the contrast factor C is equal to -10 log (I.sub.2 /I.sub.1) where I.sub.1 and I.sub.2 are respectively the maximal and minimal transmitted intensity.
However, rotation of the etalon reduces the spectral bandwidth and the amplitude of the transmitted optical signals.
FIG. 4 shows a simulation of the variation in the amplitude and the spectral bandwidth of the transmission peak for different Fabry-Perot interferometer rotation angles.
The characteristics 40 through 43 show the transmission peaks at Fabry-Perot etalon rotation angles .alpha. of 0, 2, 4 and 6.degree. when the filter is tuned to input optical signals of intensity I.sub.0.
For normal incidence (.alpha.=0) the transmission peak passes almost all of a selected wavelength, i.e. there is little attenuation. Also, the sides of the peak 40 are steep and this ensures good rejection of optical wavelengths adjacent that to which the filter is tuned (narrow spectral bandwidth).
For tuning with a rotation angle of 2.degree. (characteristic 41), however, attenuation of the transmitted intensity is accompanied by spreading of the spectral bandwidth. This phenomenon becomes more accentuated as the angle .alpha. increases and causes crosstalk between closely adjacent channels.
To a first approximation (normalized unity amplitude channels of infinitely small spectral bandwidth), the crosstalk D is equal to 2. I.sub.1 /I.sub.0 where I.sub.0 is the maximal intensity of the transfer function and I.sub.1 is the intensity at a phase angle representing the position of the adjacent channel.
This crosstalk distorts the optical signals at the filter output and is shown by the curve 50 in FIG. 5. The interferometer rotation angle .alpha. in degrees is plotted on the abscissa axis and the crosstalk in percent on the ordinate axis.
Because of this crosstalk, if the optical signal is modulated by a digital signal significant closing of the eye pattern occurs for optical signals at wavelengths which require a large angle .alpha. to filter them when the optical signal density is high in the free spectral range and the transmission error rate increases.
To avoid this problem it is necessary to restrict the number of different wavelength channels in a given free spectral range. With a sufficiently wide gap between two adjacent wavelengths interchannel crosstalk can be limited and an acceptable error rate preserved.
One palliative measure is to increase the reflectivity of the Fabry-Perot etalon but this is accompanied by a decrease in the spectral bandwidth and so is not suitable for filtering input signals which have a variable spectral bandwidth. Using direct amplitude modulation, for example, spurious frequency modulation (chirp) can increase the spectral bandwidth of the channel to be filtered by a non-negligible amount.
The overall spectral bandwidth/free spectral range ratio can also be enhanced by using two Fabry-Perot filters in cascade. Increasing the spectral bandwidth/free spectral range ratio increases the resolution and therefore reduces crosstalk. Energy losses are doubled, however, and the spectral bandwidth is reduced. Thus this solution is no more suitable for optical signals having a significant spectral bandwidth. This solution is also more costly.
One object of the present invention is to remedy these drawbacks.
In more precise terms, one object of the invention is to reduce the crosstalk between a filtered channel and a channel near the filtered channel in order to reduce the error rate for transmission of optical signals in a system such as an optical switching system, for example. Any such reduction in crosstalk in the tunable range (FSR) would make it possible to enhance the contrast between the signals and thus the resolution of the filter.
Another object of the invention is to preserve a minimal spectral bandwidth over all of the tuning range, i.e. for different interferometer inclination angles.